What is Electropolishing and what is it used for?

  • In Layman's terms electropolishing is a material removal process using an electrically powered chemical process called electrolysis.
  • The amount of material removed during the process is very small - for polishing purposes only
  • Electropolishing is often used with surgical instruments, medical tools and equipment related to food processing, polishing the top of the surface
  • It is not always possible to remove rough scratches on the surface using electropolishing

electropolished engineElectropolishing is a material removal process using an electrically powered chemical process called electrolysis. In this process the amount of material being removed is very small as it is done for polishing purposes only. A negatively charged copper electrode and positively charged workpiece are dipped into a solution of fast flowing electrolyte solution. When connected to the battery power the working surface starts to dissolve into the solution.

The process of Electropolishing

The electropolishing process involves the removal of material via diffusion from the workpiece into an electrolyte solution. The elements required are as follows:-

  • A tank which contains different acid based electrolytes (the selection of acid is dependent on the material being polished)
  • Copper cathodes
  • Workpiece working as anode in the system.

The power source is usually a direct current. The negative point is connected to two or more copper cathodes and the positive terminal to the workpiece. At this moment the workpiece is surrounded by copper cathodes. The system also contains a heating system as the electrolytes need to be at a high constant temperature. There is also a pump or stirrer to move the electrolytes during the process.

Material diffused in this process does not deposit onto the cathode. It simply remains in the electrolyte solution – a proper sludge tank and filter mechanism is also part of the system. The sludge tank takes input from main tank and the suction pump forces the fluid through the filter. After filtration the fluid moves to the heat exchanger where it is reheated before running back into the system. As this process is controlled by the direct current, the amount of material being removed depends on the current intensity.

The graph below shows the current intensity relation with anode potential. Between the area from zero to A only smoothing of a work piece can be done and between B and C the process of polishing occurs.

Applications for Electropolishing

  • Process is typically used as a pre-process for electroplating where it helps create a good quality adherence
  • Electro-plating is widely being used for high quality room chairs, tables and waste bins
  • For the polishing of highly sensitive and small sized instruments which includes surgical instruments, medical tools and equipment related to food processing
  • This process is also used to remove sharp edges from small tools and instruments which cannot be removed by hand filing/conventional tools
  • Electropolishing is also a common post-process for many products where there has been distortion due to processes like nitriding, annealing, welding, soldering and carburizing
  • The process can also be used to increase the reflectiveness of a product’s outer surface
  • When looking to remove the stressed/unwanted hardened layer from a product material, this is perfect – it can also improve the life of the product
  • Micro machining of metals and different alloys

Materials

Any material which can conduct electricity can benefit from this process. Some common materials are:-

  • Steel
  • Nickel Alloy
  • Copper Alloy
  • Titanium Alloy

Mechanical Design Guidelines

  • Electropolishing affects geometric features like sharp corners which cannot remain sharp after the process. There are some limitations such as a radius of about 0.05 mm and hole size of about 0.1 mm diameter. Note that the hole does not remain straight or perfect after this process
  • With wall roughness of about 0.2 to 12.5 micro meters, excelled surface conditions are achieved using this process. The tolerance limit which can be achieved is about 0.013 mm minimum to 0.5 mm maximum depending upon how long the process lasts

Electropolishing Process Variations

The process of electropolishing is basically controlled by seven different parameters which directly impact the results achieved. Those parameters are as follow:-

  • Workpiece material
  • Workpiece surface condition
  • Density of the current applied
  • Original surface roughness
  • Voltage being applied across the workpiece
  • Type and nature of acid being used
  • Temperature of the electrolytes during the process

For a particular material which needs to be polished using this process there are some specific acids which can be used. The current and voltage density will be set as a consequence of the surface conditions.

Economics of the Process

  • As the electropolishing process has a moderate production rate it is very economical for small to medium production quantities. However in some circumstances high production volumes are achievable to some extent
  • A typical removal rate using this process is about 50 to 250 cubic meters per second with linear penetration of 0.15 mm per second
  • Due to high power consumption during the process, its uses are often limited to high value products
  • Process has short setup time but poor material utilisation
  • Electrolytes can be harmful to the environment if not disposed of properly
  • Complete process can be easily automated making it easier to use potentially dangerous acids
  • Equipment cost is very high but the labour cost is low to moderate – depending upon the complexity

Advantages of Electropolishing

  • Electropolishing process can be used to treat multiple products at the same time
  • Or multiple amounts of the same product
  • This process can be used safely with materials that are susceptible to heat damage
  • Excellent alternative for all those components affected by thermal processing
  • This process allows the polishing of very complex shapes – often to complexity levels which cannot be polished by any other process

Disadvantages of Electropolishing

  • Electropolishing is not capable of removing surface defects like the seam or the inclusion of non-metal in a metal component
  • If a component consist of more than two alloys (the multiphase alloy) where one alloy does not support anode diffuse, then the process will not work
  • Any rough scratches on the surface of the component cannot be removed using the electropolishing process
  • Products which contain a high amount of silicon, sulphur and lead can be troublesome
  • Electropolishing can remove small scratches but it is not effective on a wavy surface
  • This process is heavily reliant on the condition of the base metals

3 Responses to What is Electropolishing and what is it used for?

  1. andyappan says:

    Low carbon Automotivea. 50 % co2e of BS VI / Euro 6  by Andy Bearing- double speed for same ton-hp-fuel-vehicle. 2 x kmpl. convert by 20% costb. 25 % co2e of BS VI / Euro 6  by Andy Bearing- double speed for same ton-hp-fuel-vehicle. 2 x kmpl + Muthukal 1/2  Engine- volume, 1/2 fuel, 1/2 Co2e for same ton-hp-vehicle. 4 x kmpl. convert by 50 % cost

  2. andyappan says:

    can it be reversed for electro plating of metal ?

  3. andyappan says:

    1 name some electrolyte for copper, nickel, carbon, , aluminium, chrome,2 can a liquid metal be used as cathode? sodium, potassium, mercury

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